An impedance of an antenna in a high-frequency (or a radio-frequency) signal transmitter used in wireless communications is influenced by variances in the environment surrounding the antenna (for example, the approach of an automobile, or physical contact by a user, etc.). Variation in the impedance may cause a mismatch between the impedance of the antenna and that of a power amplifier supplying a signal to the antenna, and may adversely affect the performance of the wireless communications. In particular, such mismatch may generate a reflected signal component flowing from the antenna to the power amplifier, causing interference among signals within the signal transmitter, and a reduction in performance or efficiency of the signal transmitter.
As is known, an impedance matching between a power amplifier and an antenna (or a load) is an important parameter in achieving a linearity of a signal transmitter and ensuring a normal operation thereof. However, the signal transmitter linearity is influenced by a variation in the impedance of the antenna. Although an antenna for use in a general circuit has a particular impedance, it may be affected by and vary with changes in the surrounding environment (e.g., due to an approaching automobile which reflects signals transmitted from the antenna).
One way to reduce a variation in the impedance of an antenna to address a deterioration in the linearity of a signal transmitter and in the efficiency thereof, is to add an isolator between the antenna and the power amplifier to apply a substantially uniform load impedance to the power amplifier. Although the isolator may render it possible to provide a substantially uniform load impedance to the power amplifier, it may not be always practical to provide an isolator in a mobile communications device due to its size and/or extra cost.
What is needed therefore, is a method and apparatus to providing improved linearity in a signal transmitter that overcomes at least the deficiencies described above.